Restoration of CBCT Images by Deformable Registration for Plan Evaluation and Replanning

L Archambault*, L Gingras, CHU de Quebec, Quebec city, QC

Purpose:To use a simple deformable image registration (DIR) scheme to correct CBCT images in order to use them for treatment plan assessment and replanning.

Methods:A dataset of five prostate patients with weekly CBCT scans was available for this study. Contours of soft-tissue organs were made on the planning CT as well as on each CBCT. The following procedure was applied: (1) a mask was automatically generated to exclude certain area from the registration (table couch, CBCT truncated region due to limited field of view); (2) a B-spline based DIR algorithm was used to register each plan CT to daily CBCTs; (3) CBCT regions where air appeared (i.e. air is seen on the CBCT but not on the plan CT) were automatically segmented and excluded from the registration because of the poor performance of DIR algorithms in such cases.

Results:Prior to DIR, the average differences in Houndsfield units between CT and CBCT were 26, 7, 107, 125 for the prostate, bladder, seminal vesicles and rectum respectively. After DIR these differences were respectively reduced to 5, 5, 4, 2. The external contours of the patients were assessed using Dice's coefficient. On average the Dice coefficient between CT and weekly CBCTs was 0.92. After DIR this value was improved to 0.97. Furthermore by registering the larger CT to shorter CBCT (i.e. 64 slices only) we obtain realistic scattering media outside the immediate CBCT field of view. The whole process takes about 4 minutes although no explicit speed optimization has been done yet.

Conclusion:This technique has several advantages for plan assessment and/or replanning purposes: it combines the accurate Houndsfield numbers from the plan CT with the daily anatomy of the CBCT, preserve the external contour and provide realistic scattering media outside the CBCT field of view.